Volume-rate respirator system and method

ABSTRACT

A respiration unit for supplying a preselected limited volume of breathing gas to a patient at limited positive pressures, and at a preselected limited flow rate is disclosed. The system includes an air delivery cylinder having a drive weight which is free to move up and down. A diaphragm connected to the weight divides the cylinder into an upper control chamber and a lower delivery chamber. A vacuum source is connected to the control chamber through a first solenoid-controlled valve to regulate the lifting of the weight, and a second solenoid-controlled valve regulates the bleeding of air into the control chamber to release and regulate the rate of fall of the weight, thereby driving breathing gas under pressure out of the lower chamber. A control system regulates the lift and lower cycles of the drive weight to provide the required operation of the system, with various sensors being provided to allow patient control of the system under selected conditions, and to assure patient safety. The control system incorporates, among other things, means for providing a variable inspiratory-expiratory ratio, for permitting patient assisted breathing, for mixing oxygen with air or anesthetic gases, for periodically altering the breathing pattern, for permitting selection of positive or negative end expiratory pressure, for allowing selection of the breathing waveform, and for providing accurate displays of essential parameters.

United States Patent 1191 Eyrick et al.

[451 Sept. 16, 1975 1 1 VOLUME-RATE RESPIRATOR SYSTEM AND METHOD [75] Inventors: Theodore B. Eyrick, Reading; Allen C. Brown, Acton; Neil R. Hattes, Danvers, all of Mass.

[73] Assignee: Chemetron Corporation, Chicago,

[22] Filed: Oct. 2, 1973 [21] Appl. No.: 402,677

52 US. Cl. 128/l45.8; 128/145.6; 417/328 51 1m.(:1. A61M 16/00 [58] Field ofSearch 128/142, 145, 145.5, 145.6, 128/1457, 145.8, 188; 222/3; 417/328, 392,

Primary Examiner-Richard A. Gaudet Assistant Examiner-Henry J. Recla Attorney, Agent, or Firm-Jones, Tullar & Cooper [5 7 ABSTRACT A respiration unit for supplying a preselected limited 56 ANESTH. l 3 so 54 DUST FILTER 114 l E LIFT vacuum,

VALVE PUMP volume of breathing gas to a patient at limited positive pressures, and at a preselected limited flow rate is disclosed. The system includes an air delivery cylinder having a drive weight which is free to move up and down. A diaphragm connected to the weight divides the cylinder, into an upper control chamber and a lower delivery chamber. A vacuum source is connected to the control chamber through a first solenoid-controlled valve to regulate the lifting of the weight, and a second solenoid-controlled valve regulates the bleeding of air into the control chamber to release and regulate the rate of fall of the weight, thereby driving breathing gas under pressure out of the lower chamber. A control system regulates the lift and lower cycles of the drive weight to provide the required operation of the system, with various sensors being provided to allow patient control of the system under selected conditions, and to assure patient safety.

The control system incorporates, among other things, means for providing a variable inspiratory-expiratory ratio, for permitting patient assisted breathing, for mixing oxygen with air or anesthetic gases, for periodically altering the breathing pattern, for permitting selection of positive or negative end expiratory pressure, for allowing selection of the breathing waveform, and for providing accurate displays of essential parameters.

52 Claims, 19 Drawing Figures PRES. l REG. rp

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r ATENTEB SEP 1 5 75 SHEET x 2 1 n w? u M 3% 7 E2: 555 5E M352 M 532 E5 l I F L I I I l l l l l i I I l. I L l I I l l I l I I l l I l l l I I I I l l I l I V Xvi mam 9m M58 5 wwzwm mum J EEG Lb sEoz u VOLUME-RATE RESPIRATOR SYSTEM AND METHOD BACKGROUND OF THE INVENTION The present invention relates, in general, to respiration systems, and more particularly to such a system which inherently provides a safe, reliable operation and which is easily controlled, highly flexible, and capable of a great variety of operational modes which permit a wide choice of functions for maximum effectiveness in use.

This application is related to copending application Ser. No. 402,679, of Theodore B. Eyrick and Allen C. Brown, filed on even date herewith and entitled Breathing Gas Delivery Cylinder for Respirators and application Ser. No. 402,678, of Theodore B. Eyrick and Neil R. l-Iattes, filed on even date herewith and entitled Respiration Ratemeter, the disclosures of which are hereby incorporated herein.

Numerous types of respiration devices and systems have been developed in the prior art, and have at one time or another been on the market. Such systems have all generally had characteristics and features which met special needs or which overcame specific problems, and thus were of value to the art in specific circumstances. But, unfortunately, many of the prior art systems had flaws which made them inappropriate for use insome cases, or which made them unreliable, difficult to control, or inaccurate, and thus the search has continued for an improved respirator which would overcome such difficulties.

The basic types of respirators are well known, having been in clinical use for many years and often discussed, both as to advantages and shortcomings, in the technical literature. Some of the respirators now in use are described by W. W. Mushin, L. Rendell-Baker, P. W. Thompson and W. W. Mapleson in their book Automatic Ventilation f the Lungs, 2nd Ed., F. A. Davis Co., Philadelphia, Pa. (1969), and by W. T. Heironimus in Mechanical Artificial Ventilation, C. C. Thomas, Springfield, Illinois 1967), as well as in other publications.

Broadly speaking, respirator devices can be classified by the parameter which controls the cycling of the machine: pressure, time, or volume of gas, and many currently available machines can be so classified. A pressure-cycling machine allows air to be delivered to a patient until a preset pressure is reached, at which time the control system closes the valve controlling air flow. A disadvantage of the pressure-cycled machine is that it will deliver varying quantities (tidal volumes) of air when the pulmonary back pressure or the compliance of the system changes. This requires that the ventilation be very closely monitored to insure that the gas level of the blood does not fall outside desired limits. Generally, too, such systems have relatively low pressure ca pabilities and present problems in controlling the amount of oxygen delivered to the patient.

A time-cycled machine sets the time for inspiration and expiration, so that the volume of air actually delivered becomes a function of flow rate. Since the flow is limited by pulmonary resistance, changes in this factor leads to variations in the volume delivered. Most such systems also have relatively low peak pressures.

Volume-controlled machines provide a relatively constant tidal volume delivery, except, under very high pulmonary resistance conditions, where losses due to the compressibility of the gas becomes a factor. These devices generally have high pressure and flow capability, but may be limited in the particular features made available in a given machine. Although volumecontrolled machines provide the advantage of delivering a desired quantity of air on each cycle, the utilization of this concept has not been without problems in prior art machines. For example, in a volumecontrolled machine it is essential that the measurement of volume be very accurate to insure that the machine functions properly. However, prior machines have not been reliable in this regard, and because of compliance in the machine itself, difficulties in obtaining an accurate determination of the location of a movable piston or like air drive mechanism, and sluggish mechanical or electrical control systems, it has not been possible to provide a volume system that would repeatedly provide a selected quantiity of air for a patient.

Another difficulty encountered with volumecontrolled systems is the likelihood of encountering excessive pressures, and such systems thus can produce a considerable safety problem. Where a gas delivery machine is arranged to sense and be controlled by the volume of gas being delivered, high pressures can appear, and it then becomes necessary to provide often complex sensing and control systems to guard against injuring the patient. However, the fact that dangerous pressures can be produced is a safety hazard in itself, for failure of the control system can result in damage. For example, pressure relief valves are commonly used in such systems to regulate the maximum pressure that can exist; but the operational characteristics of such valves can change over a period of time, and if one should fail at the wrong time, a patient could be injured.

In a volume system, it is common to provide a be]- lows or other air container which will receive the gas to be delivered to the patient. The container is then compressed mechanically or pneumatically to discharge the air to the patient when the inhalation portion of a breathing cycle is reached. In such systems it is customary to fill the container to its maximum volume prior to inhalation, and to then terminate the discharge when the desired volume has been delivered. A real danger exists with this arrangement, however, for if the control system should fail during the discharge stroke, the full volume of air the machine is capable of delivering can be discharged into the patients lungs, and can do irreparable harm.

A volume controlled system which relies on sensing the delivered volume and controlling the end point of the delivery stroke may also be inaccurate because of the overshoot that will occur after the stop signal has been given. Inertia within the mechanical delivery apparatus and lost motion in the controls may result in the mechanism moving past the desired stop point, and this can produce an inaccuracy in the delivery of the breathing gas.

In general, then, it can be said that although prior respirator or ventilator devices have been satisfactory, nevertheless, they have been too inflexible and have not been capable of safely meeting the needs of various patients in various circumstances. A patient whose breathing must be completely controlled, for example, requires different machine characteristics than a patient whose breathing is merely being machine assisted. Often a patient whose breathing has been machine controlled for an extended period becomes dependent upon the machine, and cannot breathe without it. Such a patient must be gradually withdrawn from the machine, and thus flexibility in operation to allow shifting from machine to patient controlled breathing is high desirable. A machine that does not have flexibility and reliability, then, cannot meet the varying needs of patients.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a respirator unit which is flexible, reliable, easily controlled, and, above all, safe.

It is a further object of the invention to provide a respirator system which is easy to use, compact, and accurate, and which provides a variety of features to enable safe and convenient delivery of breathing gas to a patient.

In accordance with a preferred embodiment of the present invention, a respirator system is provided which is mechanically arranged and electrically controlled to produce an accurate and repeatable delivery of a preselected limited volume of breathing gas to a patient at limited positive pressures and at preselected limited flow rates. The system responds to a variety of sensed parameters to regulate the respirator apparatus and provide total and adjustable control of the breathing gas supplied to a patient, in accordance with the desires of the operator. This is accomplished through an air delivery unit which comprises a vertically positioned cylindrical housing containing an axially extending hollow shaft which forms a guide for a movable weight that is mounted, by means of suitable bearings, on the shaft. A follower chain extends through the hollow central shaft and is attached to the weight and to an external potentiometer, whereby motion of the weight within the housing causes a corresponding variation in the output of the potentiometer to provide an indication of the exact position of the weight within the cylinder. A rolling diaphram seals the circumference of the sliding weight to the inner surface of the cylindrical housing into an upper and lower chamber. The chambers are sealed from the central shaft by means of upper and lower bellows which surround the shaft and are secured between the sliding weight and the upper and lower ends of the housing.

A vacuum pump is connected to the upper chamber through a solenoid-operated control valve, while the lower chamber is opened through an inlet check valve to a supply of breathing gas and through an outlet check valve to the patients lungs, the valves serving to insure the proper direction of flow. Application of a vacuum to the upper chamber allows the breathing gas, which is at approximately atmospheric pressure, to enter the lower chamber and lift the sliding weight upwardly to a preselected position, the breathing gas filling the expanding lower chamber. The location of the weight is monitored by the potentiometer so that when it approaches the desired level, corresponding to a desired volume of gas in the lower chamber, the vacuum control valve can be closed. The breathing gas may be atmospheric air, oxygen, an anesthetic breathing gas mixture, or a combination of those in accordance with the needs of the patient.

Also connected to the upper chamber is a bleeder valve which is variable to allow the vacuum in the upper chamber to be relieved at a predetermined maximum rate. This allows the weight to move downwardly under the force of gravity to compress the lower chamber and force breathing gas out of its outlet at a rate limited by the setting of the bleeder valve, and by the back pressure produced in the lower chamber.

The raising and lowering of the movable weight within the cylinder, and the consequent expanding and contracting of the lower chamber, produces a periodic flow of air from the chamber through suitable conduits, filters, humidifiers, and the like to a conventional breathing manifold, which may be connected to a mask or inserted tube, for supplying breathing gas to a patient in accordance with a preselected, adjustable breathing cycle. As soon as the cylinder has completed the delivery of gas, the weight is lifted to the preselected volume location in readiness for the next breathing cycle, and is held in that location until the control system calls for the delivery of air.

As is known, a breathing cycle includes an inspiration mode, or phase, when the patient inhales or is caused to inhale by the respirator, an expiration mode, when the patient exhales, and an inflation hold mode, which occurs between inspiration and expiration, when the patient is in effect holding his breath. The time relationship of these three modes may be referred to as the breath waveform, and in the present system this waveform may be adjusted to provide the required breathing pattern for the patient. In the inspiration mode, which is at the start of a breathing cycle, the cylinder is operated to deliver breathing gas at a rate, volume, and pressure that is determined by the breath waveform and the setting of the machine controls. During the hold and exhalation modes, the cylinder drive weight returns to its upper position and a check valve is closed to prevent exhaled gas from returning to the cylinder. Following the end of the hold mode, if any, the exhalation mode occurs when an expiration path is opened through the conventional breathing manifold to atmosphere. During the exhalation mode, if it is desired to assist the patient to exhale, or if it is desired to produce a negative end expiratory pressure (neep), a flow of air is generated through a venturi arrangement which then serves to draw air out of the patients lungs. When a positive end expiratory pressure (peep) is desired during the exhalation mode, the expiration balloon valve within the conventional breathing manifold is cycled to maintain the preselected level of positive end expiratory pressure.

The system is controlled by a circuit which effects the energization of the various solenoid-operated valves in accordance with the condition of the patient, of the respirator itself, or in accordance with the values of adjustable controls and sensors. Several of these adjustable controls and sensors are photosensors which respond to the system pressure at predetermined locations to operate the solenoids in accordance with a preset pattern or in response to emergency conditions. Thus, although the system is operable on a fully controlled basis, wherein the patient plays a passive role and the machine produces the necessary inspiratory and expiratory breathing patterns, it is also operable on a patient demand basis, wherein activity by the patient in attempting to breath on his own overrides the preset rate control. The activity of the patient in trying to inhale during an expiratory mode, for example, may be sensed as a change in pressure within the breathing manifold; and this change may be used to override the preset rate pattern and allowthe patient to inhale before completion of the programmed exhalation. The degree of patient activity required to override the system operation may be preset'by a sensitivity control which may be adjusted to provide the override at a se lected pressure level. Other sensors detect low and high pressures within the system to insure proper cycling, and established limits to provide an extra measure of patient safety. Various manual overrides are also available in the system to permit the operator toinitiate a breathing cycle, or to produce a sigh, at any time in place of the automatically recurring breathing and sigh cycles provided by the system. Various other automatic and manual controls will become apparent from the more detailed description which follows. i

The operation of the system is carriedout by means of a logic which responds to the various sensors and control switches to provided output signals which serve to activate or deactivate corresponding solenoid valves, indicator lights, alarms, and the like. The logic control circuitry of the system may be roughly divided into three general functions; basic control, power, and alarm and volume control. The basic control circuit regulates the normal cycling rate of the machine and the aborting of a cycle in response to an alarm or an override caused by patient demand. The control also responds to an exhale assist signal and permits manual as well as automatic regulaction of normal and sigh cycles. The normal cycling of the machine is under the control of an astable multivibrator within a normal rate control circuit which initiates the inspiratory mode of the machine at preset intervals to start the successive machine cycles. The output pulses from the normal rate control energize a pulse generator to create an execute cycle pulse which is used in conjunction with the rate control signal to operate the system in accordance with the predetermined parameters. The operation of the breathing cycle, which includes the inhale, hold, and exhale modes, is automatic, unless overridden in response to monitored conditions, whereby a positive control of the breathing cycle and of the inspirationexpiration ratio is provided.

The power control circuitry responds to the control signals to drive the solenoid-controlled valves, provides a clearing signal for clearing the logic circuitry immediately following the start up of the system upon initial application of power, provides machine synchronization, monitors power failures and powers the audible and visual alarms for the system.

The alarm and volume control circuitry in the preferred embodimemt of this system monitors such machine conditions as' proper cycling of the gas delivery cylinder to insure that the full volume is being delivered, oxygen ratio in the breathing gas, pressure limits within the system, and compares the volume control setting of the machine with the rest position of the gas delivery drive weight at the end of its upstroke. This circuit portion also provides condition indicators, and incorporates a ratemeter for responding to successive inhalations to provide a continuous breathing rate indication.

A respirator system thus constructed overcomes many difficulties encountered with prior art systems and provides a highly accurate, repeatable delivery of preselected volumes of breathing gases at preselected rates, with a high degree of safety. Because of the use of a delivery cylinder having a rolling diaphragm arrangementseparating an upper control chamber from a lower delivery chamber, a delivery chamber having a very low compliance is provided, thereby permitting accurate control of the volume of the gas being delivered to the patient, and permitting limitation of the pressure within close tolerances.

The use of a sliding weight-to provide the driving force on the delivery stroke of the cylinder produces a distinct improvement over prior devices in that the air delivery system does not rely-on complex control systems to protect the patient. This is due to the fact that the maximum pressure that can be generated in the delivery chamber under any condition is that produced by the force of gravity acting on the weight. By selecting this weight, in the design of the cylinder, to have a mass which will produce a maximum pressure within known limits even under free falling, or no-control, conditions, the system may be made selfpressure limiting, thereby providing a large margin of safety for the patient. Again, because it is the operation of the moving weight under the influence of gravity, but restricted by the bleeder valve, that produces-the outward flow of gas to the patient, the cylinder is limited as to the maximum rate of flow that can be produced, thereby further improving patient safety with fewer components than are required in prior art systems. I

Since the weight remains constant as it moves down the central shaft of the cylinder during the delivery stroke, the flow of gas to the patient is unaffected by cylinder compliance and other variations that occurred in prior art systems. Furthermore, the moving weight arrangement of this invention provides a simple, highly accurate, repeatable method of selecting and delivering a desired volume of breathing gas. By regulating the position taken by the weight at the end of its upward, or loading stroke, the volume of breathing gas drawn into the delivery chamber is accurately known, because of the low compliance of the chamber. Further, the location of the weight, and thus the volume of the delivery chamber, is monitored prior to the delivery stroke so that if an error has occurred in the control system it can be recognized before the delivery portion of the cycle begins. Because the delivery stroke always ends adjacent to the bottom of the cylinder, discharging virtually all the air stored in the delivery chamber, only the preselected volume can be delivered.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and additional objects, features, and advantages of theinvention will be more fully appreciated when considered in the light of the following detailed description of a preferred embodiment thereof, selected for purposes of illustration and shown in the accompanying drawings, in which:

FIGS. 1 and 2 provide a diagramatic illustration of the respirator system of the present invention, includ ing the air delivery cylinderand accompanying pneumatic controls;

FIG. 3 illustrates the relationship of FIGS. 1 and 2;

FIGS. 4 through 7 provide a functional block diagram of a preferred control arrangement for the respirator system of FIGS. 1 and 2;

FIG. 8 illustrates the relationship of FIGS. 4-7;

FIGS. 9 through 17 provide block diagram of the logic circuitry used in the respirator system of the present invention, corresponding to the functional diagram of FIGS. 4-7; 

1. In a respirator system, a breathing gas delivery cylinder including a housing and piston means dividing said housing into an upper control chamber and a lower delivery chamber; means for producing a vacuum in said control chamber for lifting said piston means to expand said delivery chamber; means for releasing said vacuum to lower said piston means and thereby compress said delivery chamber; control means for periodically and alternately energizing said vacuum producing means and said vacuum releasing means for drawing breathing gas into said delivery chamber and for discharging said breathing gas; delivery means connected to said discharge means for directing said breathing gas to a patient.
 2. The system of claim 1, wherein said delivery means comprises a patient breathing manifold, and conduit means connecting said discharge means to said manifold.
 3. The system of claim 2, said delivery means further including valve means in said conduit means for restricting the direction of flow of said breathing gas in a direction toward said manifold.
 4. The system of claim 2, wherein said manifold includes outlet means for receiving exhaled breathing gas from said patient, and selectively operable valve means in said outlet means for controlling the exhalation of said gas.
 5. The system of claim 4, wherein said selectively operable valve means in said outlet includes a solenoid-operated expiratory valve.
 6. The system of claim 5, wherein said selectively operable valve means in said outlet includes a balloon valve adapted to open and close said outlet means, and a solenoid-operated expiratory control valve selectively operable to expand said balloon valve to close said outlet means to prevent exhalation by said patient and open said outlet means to permit exhalation by said patient.
 7. The system of claim 6, further including a source of pressurized gas for said balloon valve, said expiratory control valve being operable to connect said balloon valve to said source of pressurized gas to expand said balloon.
 8. The system of claim 6, further including expiratory assist means for producing a pressure reduction in said outlet means for aiding patient exhalation.
 9. The system of claim 8, wherein said expiratory assist means comprises a source of pressurized gas, Venturi means, a solenoid-operated expiratory assist valve conencting said source of pressurized gas to said Venturi to provide a source of negative pressure, and means for connecting said source of negative pressure to said outlet means to produce said pressure reduction therein.
 10. The system of claim 9, wHerein said means for producing a vacuum in said control chamber comprises a source of vacuum, and lift valve means selectively operable to connect said vacuum source to said control chamber.
 11. The system of claim 10, wherein said lift valve means is a solenoid-operated valve, said control means periodically operating said solenoid valve to lift said piston.
 12. The system of claim 9, wherein said means for releasing said vacuum comprises solenoid-operated bleeder valve means connected to said control chamber and periodically energized by said control means to allow said piston to compress said delivery chamber.
 13. The system of claim 12, wherein said means for releasing said vacuum further includes a variable rate orifice for limiting the rate at which said vacuum is released.
 14. The system of claim 13, wherein said means for producing a vacuum in said control chamber comprises a source of vacuum and solenoid-controlled lift valve means operable by said control means for connecting said source of vacuum to said control chamber.
 15. The system of claim 14, further including inlet means for supplying at least a first fluid to said cylinder, and mixing means for selectively supplying to said inlet means a second fluid at a controlled rate.
 16. The system of claim 15, wherein said mixing means comprises a pressurized source of said second fluid and variable orifice means connecting said source to said inlet means, said variable orifice means limiting the rate at which said second fluid is supplied to said inlet.
 17. The system of claim 16, wherein said mixing means further includes solenoid control valve means between said source and said inlet means and energizable by said control means to allow said second fluid to be supplied to said inlet.
 18. The system of claim 17, wherein said control means is operative to supply said second fluid to said inlet only during lifting of said piston.
 19. The system of claim 18, wherein said first fluid is air and said second fluid is oxygen.
 20. In a respirator system for delivering at limited positive pressures a limited volume of breathing gas at a limited flow rate, a breathing gas delivery cylinder including a housing and piston means dividing said housing into an upper control chamber and a lower delivery chamber; a source of vacuum; a solenoid-controlled lift valve operable to connect said vacuum source to said control chamber to lift said piston and expand said delivery chamber; means for admitting a breathing gas into said delivery chamber upon expansion of said delivery chamber; solenoid-controlled lower valve means connected to said control chamber and operable to admit air at atmospheric pressure into said control chamber to lower said piston whereby the weight of said piston will compress said delivery chamber to discharge said breathing gas; means for controlling the flow of atmospheric air into said control chamber to limit the lowering rate of said piston; outlet means connected to said delivery chamber for receiving breathing gas discharged from said delivery chamber when said piston is lowered; and delivery means connected to said outlet means for directing said discharged breathing gas to a patient.
 21. The system of claim 20, wherein said delivery means further includes: a patient breathing manifold, said manifold having means for directing said breathing gas to said patient and for receiving breathing gas exhaled by said patient; and a solenoid-controlled expiration valve for opening said manifold means to permit exhalation by said patient.
 22. The system of claim 21, further including a solenoid-controlled expiratory assist valve for producing in said manifold means a reduction in pressure for aiding patient exhalation.
 23. The system of claim 22, further including control means for periodically activating said lift valve and said lower valve for drawing said breathing gas into said delivery chamber during lifting aNd for discharging said breathing gas during lowering of said piston.
 24. The system of claim 25, wherein said control means includes circuit means energizable in an inspiratory mode to operate said lower valve means to lower said piston to deliver breathing gas to said patient, and energizable in an expiratory mode to operate said expiration valve to permit said patient to exhale.
 25. The system of claim 24 wherein said control means further includes circuit means energizable in a hold mode to prevent said control means from operating in either an inspiratory or expiratory mode.
 26. The system of claim 25, further including breathing cycle control means for selectively operating said inspiratory mode circuit means, said expiratory mode circuit means, and said hold mode circuit means in timed sequence to provide an adjustable breathing waveform.
 27. The system of claim 26, wherein said breathing cycle control means includes rate control timer means for producing a train of normal rate control pulses, and execute cycle pulsing means responsive to said normal rate control pulses to initiate a breathing cycle by energizing said circuit means in said inspiratory mode.
 28. The system of claim 27 further including inflation hold control means responsive to the termination of said inspiration mode to initiate a hold mode.
 29. The system of claim 28, further including inspiration timer means responsive to the termination of said inspiration and said hold modes to initiate said expiratory mode; said expiratory mode being terminated by the initiation of the next following inspiration mode.
 30. The system of claim 23, wherein said control means includes means for selecting a desired volume of breathing gas for delivery to a patient and means for activating said lift valve when said delivery chamber contains less than said selected volume, whereby said piston is raised in said cylinder.
 31. The system of claim 30, wherein said means for activating said lift valve comprises cylinder volume control means for comparing the actual cylinder volume with said selected volume, and means for inhibiting said lift valve during the discharge of gas from said delivery chamber.
 32. The system of claim 23, further including inlet pressure sensing means responsive to the pressure of the breathing gas drawn into said delivery chamber, and alarm means responsive to said inlet pressure sensing means.
 33. The system of claim 32, further including low manifold pressure sensing means and alarm means responsive to low manifold pressures.
 34. The system of claim 33, further including cycle sensing means for monitoring the cycling of said respirator, and alarm means responsive to improper cycling.
 35. The system of claim 34, further including ratio error detecting means responsive to the ratio of inspiration time to expiration time.
 36. The system of claim 23, further including adjustable pressure limiter means responsive to the pressure of the breathing gas in said manifold for terminating the delivery of breathing gas to said patient.
 37. The system of claim 22 further including normal breathing cycle control means incorporating an adjustable rate control timer for producing a train of normal rate control pulses having a selectable repetition rate, and execute cycle pulsing means responsive to said rate control pulses for periodically activating said lower valve to initiate an inhalation mode in a normal patient breathing cycle.
 38. The system of claim 37, wherein said normal breathing cycle control means further includes inflation hold control means responsive to the termination of an inhalation mode to initiate a hold mode in the patient breathing cycle.
 39. The system of claim 38, wherein said normal breathing cycle control means includes inspiration timer means responsive to the termination of said inspiration and hold modes to activate said solenoid-controlled expiration valve to initiate an expiration mode in the patient breathing cycle.
 40. The system of claim 39 wherein the duration of each of said inspiration, hold, and expiration modes is selectable to produce a predetermined timed relationship between said modes, whereby an adjustable normal breathing waveform is provided by the respiratory system.
 41. The system of claim 37 further including automatic sigh control means for periodically overriding said normal breathing cycle control means.
 42. The system of claim 37, further including error detection means for overriding said normal breathing cycle control means in the event of malfunction in said system.
 43. The system of claim 37, further including manual sigh control means for manually overriding said normal breathing cycle control means to produce a sigh function.
 44. The system of claim 37 further including manual breath control means for overriding said normal breathing cycle control means to initiate an inspiration mode.
 45. The system of claim 37, further including volume delay means for preventing the initiation of an inspiration mode until breathing gas is available in said delivery chamber.
 46. The system of claim 45, further including manually operable sigh and breath control means for overriding said normal breathing cycle control means to enable the machine operator to initiate an inspiration mode.
 47. The system of claim 46, further including inhibit means for preventing manual initiation of an inhalation mode during an existing inhalation mode.
 48. The system of claim 41, further including first adjustable pressure limiter means responsive to the normal pressure of the breathing gas in said manifold and second adjustable pressure limiter means responsive to the sigh pressure of the breathing gas in said manifold, said first and second pressure limiter means being operative to terminate the delivery of breathing gas to said patient.
 49. The system of claim 37, further including breath assist control means responsive to patient breathing efforts to override said normal breathing cycle control means initiate an inspiration mode, said assist control means incorporating sensitivity adjustment means to vary the amount of patient effort required to initiate said inspiration mode.
 50. The system of claim 37, wherein said normal breathing cycle control means further includes inspiration timer means responsive to the termination of said inspiration mode to activate said solenoid-controlled expiration valve to initiate an expiration mode in the normal patient breathing cycle.
 51. The system of claim 50, further including pressure responsive control means for modulating the operation of said expiration valve to maintain a preselected positive end expiration pressure in said manifold means.
 52. The system of claim 50, further including pressure responsive control means for activating said solenoid-controlled expiratory assist valve for producing in said manifold means a preselected negative end expiratory pressure. 